1. Field of the Invention
This invention relates to a data reproducing apparatus for reproducing main data from a disc where the main data are previously processed from input digital data by error correction coding and recoded on the disc along with auxiliary information data which are appended to the main data at predetermined intervals in the main data.
2. Description of the Related Art
Heretofore, there has been known a signal format for a so-called compact disc (CD) as a format in which PCM data are error correction coded and added to by auxiliary information data before being recorded on an optical disc. With this signal format, PCM digital audio data as main data are error correction coded by cross-interleave Reed-Solomon code (CIRC) and added to by sub-code data as auxiliary information data.
When PCM digital audio data recorded on a compact disc CD after error correction coding in accordance with the above signal format are reproduced by a conventional playback system, 8 to 14 demodulated binary signals are reproduced, and clocks are detected or reproduced on the basis of the binary signals by a phase locked loop (PLL). The frequency of these clocks, referred to hereinafter as PLL derived clocks, is 7.35 kHz.times.588=4.3218 MHz. In the conventional playback system, the binary data are reproduced by the PLL derived clocks so as to be processed by 8-14 demodulation.
The 8 to 14 demodulated data are processed by error correction and detection on the basis of the CIRC so that the data are corrected for errors. The error corrected data are subsequently processed by digital to analog conversion and thereby converted into analog audio signals.
Meanwhile, the above mentioned error correction coded data are usually error correction decoded by mean of a RAM used as a decoder. That is, the 8 to 14 demodulated data are written in the RAM used as the decoder so as to be subsequently demodulated by controlling the readout addresses of the RAM. In general, the write addresses for the RAM are formed on the basis of the above mentioned PLL derived clocks, whilst the readout addresses are based on clocks derived from an external circuit, such as a quartz oscillator.
However, the PLL derived clocks, formed on the basis of binary signals reproduced from the disc, as mentioned hereinabove, are affected by jitters caused by disturbances, such as motor revolutions or disc offsets. Conversely, the clocks derived from the quartz oscillators are substantially free from jitters. The result is that there exists a time difference corresponding to these jitters between the PLL derived clocks and the clocks derived from the quartz oscillator.
Meanwhile, there is known a system in which data are sequentially stored in a buffer RAM during ordinary reproduction so as to be read out in the event of data failure due to readout errors or defocusing of the optical pickup by way of compensation for the data failure. That is, in accordance with this system, data are readout from the disc at a fast rate in a burst fashion so as to be written sequentially in the buffer RAM, from which the data are continuously read, while the write addresses are controlled so that the neighboring data maybe continuously stored in the RAM, by way of compensation for possible data failure caused by the readout errors of defocusing of the optical pickup.
It is noted that data writing in the buffer RAM with this system is performed at a subcode Q time of the so-called CD format, that is, at an interval of one frame or 13.3 millisecond, while data readout is effected continuously at an ordinary playback rate. For this reason, the data writing rate in the buffer RAM needs to be faster than the readout rate. That is, since it is necessary to effect data writing in the RAM at a rate faster than the rate necessary to produce playback audio signals, the rotational speed of the disc is set with the above system to, for example, twice or three to four times the reference speed.
Meanwhile, the main data, such as the above mentioned digital audio data, are constituted only by PCM data and are destitute of timing data. If, for this reason, the subcode having the time information is used and the main data are divided at the subcode timing (so-called subcode Q time) for being written in or read from the buffer RAM, reproducible division points can not be obtained due to the above mentioned jitter, such that data written in the RAM are overlapped or dropped in amounts corresponding to the jitter. That is, if the digital audio data as contiguous main data segments are combined in the buffer RAM at the subcode timing, as mentioned hereinabove, it is not possible to produce fully reproducible division points, so that sound skipping or overlapping is inevitably produced.